Technical Field
The present disclosure relates to transition metal compounds and the preparation thereof, and more specifically to first-row transition metal complexes having a tetradentate and/or pentadentate supporting ligand and the use of such complexes as hydrogenation and/or hydrosilylation catalysts.
Technical Background
In recent years, 2,6-bis(imino)pyridine (or pyridine diimine, PDI) ligands have become an increasingly utilized ligand class due to their ease of synthesis, steric and electronic modularity, and ability to coordinate to a wide range of transition and alkali metal ions. Furthermore, the capacity of these chelates to accept one or more electrons from a metal center has been well documented and metrics to differentiate varying degrees of PDI reduction have been established. This redox non-innocence has proven invaluable for the advancement of base-metal hydrogenation, hydrosilylation, and cyclization catalysts whose activity rival traditionally employed precious metal complexes.
While impressive, these achievements have overwhelmingly depended on the use of sterically demanding aryl imine substituents (ArPDI). For example, the initial preparation of an (ArPDI)Fe hydrogenation catalyst depended on the incorporation of two 2,6-diisopropylphenyl imine substituents while preliminary efforts to prepare analogues with less bulky imine substituents resulted in the formation of bis(ligand) complexes rather than catalytically relevant dinitrogen complexes.
Although reduction of the respective (ArPDI)FeBr2 starting complexes using sodium naphthalenide rather than sodium amalgam has since afforded highly active hydrogenation catalysts with smaller aryl groups, the success of this approach has remained limited to 2,6-diethylphenyl or 2,6-dimethylphenyl imine substituents.
While alkyl imine PDI substituents have allowed the preparation of asymmetric Co(I) hydrogenation catalysts, their use has not yet enabled the isolation of an (RPDI)Fe (R=alkyl) hydrogenation catalyst. Similar observations have been made for related α-diimine (DI) supported first row metal catalysts.
Two of the most popular and well-studied catalytic transformations that homogeneous transition metal complexes are known to mediate are the hydrogenation and hydrosilylation of unsaturated compounds. Hydrogenation catalysts have been heavily utilized by the chemical industry and have played a large role in the synthesis of pharmaceutical precursors and products. On the other hand, homogeneous hydrosilylation catalysts are often employed to prepare silicone-based fluids, surfactants, adhesives, sealants, and coatings.
Hydrogenation, which relies upon the transfer of two hydrogen atoms from a sacrificial substrate to, or the direct addition of dihydrogen across, a C═C, C═O, or C═N bond, has traditionally been conducted in the presence of a homogenous or heterogeneous precious metal catalyst (mainly Ru, Rh, or Pt). One well-known example of this application remains the preparation of L-DOPA following the asymmetric hydrogenation of a substituted, prochiral acetamidocinnamic acid with [Rh(COD)2]+ in the presence of a chiral phosphine, as disclosed in U.S. Pat. No. 4,005,127. Asymmetric hydrogenation has also been utilized in the preparation of other chemicals, including aspartame, N-acetylcysteine, and flamprop-isopropyl.
Hydrosilylation, which typically involves the addition of a silyl hydride across a C═C, C═O, C—O, or C═N bond, has also traditionally been conducted in the presence of a homogenous or heterogeneous precious metal catalyst (mainly Rh or Pt). For example, a variety of silicone polymers have been prepared using platinum-based complexes, such as Karstedt's catalyst (U.S. Pat. No. 3,775,452), Ashby's catalyst (U.S. Pat. No. 3,159,601), Lamoreaux's catalyst (U.S. Pat. No. 3,220,972), and Speier's catalyst (J. Am. Chem. Soc. 1957, 79, 974.).
Although widely accepted precious metal complexes are efficient at catalyzing hydrogenation and hydrosilylation reactions, several drawbacks tied to their use remain. The precious metals themselves (Ru, Os, Rh, Ir, Pd, and Pt) are orders of magnitude more expensive than their first row transition metal congeners (Fe, Co, and Ni), which are found in abundance within the Earth's crust. Likewise, manganese is a low-cost and widely available first-row transition metal.
In addition to the cost advantage associated with developing efficient first-row metal catalysts, precious metals are highly toxic and must often be scrupulously removed from the hydrogenated or hydrosilylated product before it can be released to the public, a complication that also increases the overall cost basis of the transformation. Additionally, precious metal hydrosilylation catalysts are known to catalyze the formation of undesired by-products when reacted with substrates such as allyl ethers and are often susceptible to poisoning upon reaction with trace amine or phosphine impurities.
Although well-defined Mn complexes are not widely known for their ability to hydrogenate unsaturated substrates, there has been recent interest in developing Mn hydrosilylation catalysts. Several recent disclosures describe Mn hydrosilylation catalysts that are supported by kappa3-terpyridine (U.S. Pat. Pub. No. 2011/0009565), kappa3-pyridine diimine (U.S. Pat. Pub. No. 2011/0009573), and kappa3-bis(imino)quinolone ligands (U.S. Pat. Pub. No. 2012/0130021). Related disclosures describing the in situ activation of Mn complexes bearing these ligands (U.S. Pat. Pub. No. 2012/0130106), as well as a variety of other common supporting scaffolds (Int. Pat. Pub. No. WO2013/043874), have also been filed.
U.S. Patent Pub. Nos. 2011/0009565, 2011/0009573, and 2012/0130021 also cover the Fe, Co, and Ni mediated hydrosilylation of unsaturated substrates supported by kappa3-terpyridine, kappa3-pyridine diimine, and kappa3-bis(imino)quinolone ligands, respectively while U.S. Pat. Pub. No. 2012/0130106 describes the in situ activation of Fe, Co, and Ni precatalysts in the presence of the same ligand frameworks.
In a similar disclosure, the in situ activation of Fe, Co, and Ni precursors in the presence of a range of ligands has recently been filed as Int. Pat. Pub. No. WO2013/043912 and individually for Fe (Int. Pat. Pub. No. WO2013/043912), Co (Int. Pat. No. WO2013/043783), and Ni (Int. Pat. Pub. No. WO2013/043875) precursors.
There has also been recent interest in the development of well-defined Fe, Co, and Ni catalysts for the hydrogenation of olefins and alkynes. In U.S. Pat. Pub. No. 2013/0079567, tridentate ligands with chiral carbon-containing imine substituents (that may or may not be cyclometallated) have been reported to mediate the asymmetric hydrogenation of prochiral olefins. U.S. Pat. Pub. No. 2013/0079567 also covers non-asymmetric Fe, Co, and Ni hydrogenation catalysts bearing a kappa3-bis(imino)pyridine (or related) ligand.
There is a continuing need in the art for improved first-row transition metal complex catalysts that can mediate the hydrogenation or hydrosilylation of unsaturated substrates with high efficiency under mild conditions.